The present invention relates to an optical memory system and more particularly to an optical memory system wherein the memory write is carried out by the use of a change of electrical characteristic in a memory medium due to the light irradiation and the recorded data is erased by the application of a bias voltage across the memory medium.
In the prior art, optical memories have been extensively used for image input, image recording, and other forms of data storage. Among these prior art optical memories, those utilizing compounds containing chalcogenide atoms such as Cd, Se or S are widely used because it allows repeatable write and erase. In optical memories utilizing chalcogenide compounds, data is written and erased by means of a laser. However, these optical memories based upon chalcogenide compounds require that a laser device be mechanically scanned during the memory write and erase cycles. In addition, the chalcogenide compound used in the optical memory is fatigued by the repeated heating by the laser, and the written data can only be held for a few hours.
There has also been proposed a co-planar type optical memory comprising a substrate, a doping modulated multilayer film made of amorphous semiconductor formed on the substrate, and two opaque electrodes provided on the surface of the multilayer film. This co-planar type of optical memory cannot be erased except by heating the device because a potential barrier perpendicular to the surface of the multilayer film cannot be controlled from the outside. Further, the recording density is low because the electrodes are arranged in a horizontal direction with respect to a surface of the multilayer film.
It is accordingly an object of the present invention to provide a novel optical memory which permits storage of information by means of a change of electrical characteristic due to exposure to light and permits erasure of the information by application of a bias without heating.
It is another object of the present invention to provide a novel optical memory which enables the data write and erase without using a laser whereby the whole surface of the device can simultaneously be written.
It is yet another object of this invention to provide a novel optical memory which enables the storage of written or recorded memory for many hours.
It is yet another object of this invention to permit the production of novel optical memories having a large area and high recording density.
Briefly, according to one embodiment of the invention, an optical memory device is providing for storage of information. A storage means is utilized which changes for an extended period from a first value of an electrical characteristic to a second value of the characteristic in response to exposure to activating light of a selected frequency and which returns to the first value in response to application of a restore bias voltage across the storage means. A plurality of electrodes is coupled to the storage means, at least one of which is transparent to the light. The electrodes are configured such that the storage means is sandwiched between the transparent electrode and at least one other of the plurality of electrodes. In one alternative embodiment, the storage means comprises a doping modulated multilayer amorphous semiconductor.